The present invention relates to an intravascular implant permitting radial dilatation of the arterial walls. These implants or dilatators are known as xe2x80x9cstentsxe2x80x9d in the field of transluminal angioplasty. Transluminal angioplasty consists in treating diseased regions of the arterial system by the introduction of apparatus, particularly catheters, along natural routes. This permits localized intervention without having to have recourse to conventional surgical interventions which have, because of their seriousness, numerous drawbacks for the patients. This technique is particularly used when a constriction or stenosis of the arteries is diagnosed. There is then introduced through the femoral artery a catheter provided at its distal end with an inflatable angioplasty balloon. This catheter is then pushed and guided, with radioscopic monitoring, through the arterial system to the diseased region of the artery. Once this region is reached, the balloon is inflated to dilate the constricted region of the artery. This operation is repeated until it is determined, with the help of the radioscopic monitoring means, that the artery again has a diameter sufficient to ensure an acceptable blood flow. These interventions however have certain drawbacks. Thus, clinical observations show that in about a third of the cases treated, the artery again retracts over a period of time comprised between several days and several months. This phenomenon, which is called xe2x80x9crestenosisxe2x80x9d, requires a new intervention in the diseased artery either by the same method, or by more serious surgical techniques.
In an effort to solve this problem, it has been proposed to implant permanently in the artery dilatators or xe2x80x9cstentsxe2x80x9d to avoid repeated contraction. These implants usually have a tubular structure open at its ends so as not to disturb the blood flow. These devices, independently of their particular structures, generally have the following characteristics: they are radially extensible from a first diameter, permitting their introduction into the artery with the help of a catheter, to a second larger diameter corresponding substantially to the diameter of the artery. After dilatation of the artery, they are implanted in this latter and bear against the internal wall of the artery, thereby preventing, by a mechanical action, a new constriction of the artery. Once implanted, these stents have a certain resistance to radial compression and thus maintain the artery open whilst permitting blood flow. In practice, stents of two different types are used at present. The first are deformed by inflation of a balloon during their emplacement; the second stents are so-called auto-extensible. The auto-extensible stents do not require external mechanical action to pass from a first diameter during introduction, to a second larger diameter in the service position. This effect is obtained either by the use of material having a shape memory, such as Nitinol (trademark), or by a spring effect. There has also been proposed, for example by European patent EP-433 011 B1, a stent which comprises a radioactive isotope which tends to decrease the phenomenon of restenosis by radiotherapy. In other embodiments, the surface of the stent, in contact with the internal wall of the artery or the vessel, has a suitable surface treatment permitting the local distribution of antithrombogenetic chemical substances.
These devices, although having contributed to the decrease in the rate of restenosis, have nevertheless not totally solved the problem. Thus there is seen, in about 22% of the cases treated by angioplasty and the implantation of a stent, a tissue reaction which leads to increase the thickness of the internal layer of the artery. If this phenomenon is not stabilized and continues to increase, the artery becomes plugged again.
The present invention has for its object to overcome the drawbacks mentioned above, by providing a dilatation implant promoting the decrease in the rate of restenosis, in particular by its action on the internal wall of artery. Another object of the invention consists in the use of such a device to increase the shear stress at the blood/wall interface in an artery or a blood vessel. Finally, the invention also has for its object a process permitting the increase of shear stress at the level of the arterial wall. The stent according to the present invention is distinguished for this purpose by the below.